I'm at the point where I want to install some T molding for the transition from our main room to the bedrooms. I just took a look at the directions and they only show screwing the track into the subflooring. Right now I only have the moisture barrier down above the concrete subflooring, so I dont think I can just screw it down. I guess I was thinking I could glue it down, but this would require me to cut into the moisture barrier, which I'm not sure is a good idea either. I'm wondering how I should do this?

3 Answers
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There are several effective ways to hold the molding in place if you were OK with screwing it in place. One good way I would try would be to follow these steps:

Cut and fit the molding into length and shape for the spot it would get installed.

Check out the fit and see how well it fits down to the flooring on each side. Note any places where it takes a little bit of extra pressure to hold the molding down firm. Mark those spots with a pencil mark on a piece of masking tape on the molding.

Evaluate just where you would place the screws. Looking at your picture I'd say you are going to face screw it into place with brass flat head wood screws from the top. Most likely is a row of screws down the center of the Tee leg. It is also possible that you may place pairs of screws toward the two edges of the Tee leg depending on just how wide the molding actually is.

Finalize where you will place all the screw holes in the molding. When deciding on the number and spacing you may want to favor a placement the matches the areas that took extra pressure in step 2.

Drill the holes for the screws in the molding. These holes should be clearance holes with a counter sink to accommodate the brass wood screws. Do not destroy your drill bits by hitting the concrete below.

With the wood piece fitted in place make a distinct mark as to the location of each hole on the concrete. This mark can be made by sliding a spike through the hole and hammering on it with a couple of good hits to dimple the concrete below. Careful location is important in this step.

Next remove the temporarily fitted molding and set it aside. Now tape down some protective covering on your wood floor on each side door opening. The red colored Rosin paper can be good for this. The purpose is to protect the finish on your installed flooring.

Using a smaller diameter concrete drill (3/16 or 1/4" size) drill a hole down in the subfloor at each of the dimples in the concrete. Drill at least 1 1/2 inches in depth. A hammer drill can make this job much easier. The small holes are easier to drill and have stay in the proper location.

Using each of the small holes as a guide hole enlarge each hole out to a bigger diameter. I would suggest a 5/8 inch diameter. Make sure you have a shop vac handy to clean up the cement dust created by the drilling process.

Thoroughly clean the whole area of all drilling dust including inside the various holes that were drilled.

Cut lengths of hardwood dowels that will fit into each of the holes. Make each one no longer than the full diameter depth of the hole. Even a bit shorter is OK. Use 5/8 inch dowel rod if the holes were that diameter.

Next smear a thin coating of construction type adhesive onto a dowel surface. You may also want to use a small stick to smear some around the inner sides of the hole too but be careful to not put too much of the stuff. Make sure to do each dowel peg in turn.

Pound each plug into it's hole. This may take some good effort. If on first try you find the hole too tight to take the peg it may be necessary to taper off the peg some to allow it to enter the hole. (Now you see why I said to only prep the glue onto one peg at a time).

Once all the pegs are securely in place let things set a day or so for the glue to dry.

Next you are ready to remove the rosin paper from the areas where the Tee molding will lap over the flooring edge. (Do not cut the paper away with a utility knife as you would disfigure your flooring below. Fit the molding in place again.

Drill pilot holes down into the wooden pegs at each intended screw location. Note that you should carefully select the proper sized pilot drill to match the thread diameter of the brass flat head screws. Drill deeper than the length of the screws.

Once again remove the molding and vacuum out all the drilling dust. Clean the whole area thoroughly. The tape in a strip of your subfloor moisture barrier over the dowels.

Refit the mounding and screw it in place with the flat head wood screws. Putting some bar soap on the screw threads can make them way easier to install. Please note that a cross slot (Philips style) screw will be favored over a straight slot screw both for ease of installation and final looks.

Finally remove the rest of the protective floor covering and stand back to admire your good work.

I would glue the guide bar down with construction adhesive on top of the moisture barrier - given that your moisture barrier is pretty tight. Wait a day. If it is pretty solid then I would put the t-molding in with construction adhesive too... tons of glue and the t-molding with a good amount of weight on it for at least 12 hours. You do need to make sure your t-molding lays pretty flat on each side and adjust the bottom leg of it if necessary.

I would use 100% silicone. First you need to cut the moisture barrier so the silicone adheres to the concrete, Then place your T molding down. Set some weight on it then wait 24 hours. The reason why you use silicone is so your floor can expand and contract: the silicone is pliable.